It could be a bipyramid.
One word that rhymes with "cup" and starts with "B" is "blup."
Bourbon.
A compound word that starts with shape could be shape up.
a square starts with 's'
A shape that starts with G is globe.
Birthday party is one.
A square
ArcArchimedean solidAntiprismacute triangleacute trianglearcacute triangleAn arc of a circle is one exampleAn arc is a shape. An acute triangle is a shape.
Bear starts with the letter B
Easy One, Bangkok
I guess you could say that a Mobius strip is a one-sided shape. Or a circle, because there are no corners or divisions where another side starts.